Most of the applied plasma processes for surface modification and layer deposition still suffer severe constrains as the need for low pressure environment and the difficulty for easy scale-up rules that hinder their full deployment in industry. For this reason, researchers turned towards the restriction of applied plasma processes to low pressure in the early 90's by choosing the old-fashioned silent discharge, today mostly referred to as barrier discharge (BD). This non-thermal atmospheric pressure discharge was firstly applied by Siemens in his ozonizer in 1857. In recent years, this discharge type among others gained more and more attention due to the prospect of performing plasma processes usually done at low pressure at a much faster rate and without the need for expensive vacuum systems.
Of all different types of atmospheric discharges that have widely been explored, the non-thermal plasma of the BD, proved to be the most promising candidate for the treatment of temperature sensitive materials. The unique property of a non-thermal plasma is that the main constitutes, i.e. neutrals, ions and electrons, are not in thermal equilibrium, thus only electrons have mean energies of 1-10 eV whereas the overall gas temperature is close to ambient (typically around 300 K). Nevertheless, the energy of the high energetic electrons is still sufficient to initiate chemical reactions in the gas phase.
BDs have been applied in various fields such as exhaust gas purification, surface treatment or film deposition. Due to their spatial limitation to the millimeter and sub millimeter range, they appear to be beneficial for the treatment of inner surfaces of micro-structured devices as well as for the up-coming technology of plasma printing.
In plasma surface processing, BD's have primarily been applied to flat substrates of macroscopic work pieces. Otherwise, however, processing of particulate solids, i.e. granules and powders, is likely to be the most important operation in industrial production. This is most evident in chemical or pharmaceutical industry, where typically 80% of the intermediates and the majority of final agents are in solid state. Also, the polymer and plastic processing industries deal primarily with powders and granulates in the range from tens of micrometers to several millimeters. In the past, few methods have been proposed to treat powders adequately at atmospheric pressure. Recently, a device has been disclosed (DE102004048410) that allows the horizontal transport of a powder by a vibrating conveyor plate through the active zone of a filamentary surface discharge at atmospheric pressure, where silicon oxide-like material is deposited on the powder particles. A vertical, narrow-gaped BD arrangement was also reported to treat polymer particles in-situ, which means that the particles are in direct contact with the electric discharge. In both cases, filamentary discharge patterns are presumably responsible for an inhomogeneous surface modification. Such a dispersed spatial formation is the common discharge mode of an atmospheric BD. In fact, it consists of a multitude of individual discharge channels (filaments) with typical lifetimes of around 1-100 ns and radial extension of about 100 μm resulting in the aforementioned inhomogeneity of the plasma treatment. The urge to avoid inhomogeneous treatment has provoked the promising though sophisticated approach to generate so-called Atmospheric Pressure Glow Discharges (APGD), the homogeneous mode of BD. They are though very delicate with respect to their domains of existence. So far, APGDs were found to be restricted to pure nitrogen or noble gases, e.g. helium.
Since most non-equilibrium atmospheric plasmas, and especially BDs, are constricted to narrow geometries, in-situ treatment of particulate solids is tied to some major drawbacks such as uneconomically small through-put, clogging due to particle agglomeration caused by e.g. thermal hotspots or particle charging in the plasma, and substrate heating through ion bombardment in the plasma phase (important issue for temperature sensitive materials).